1. Field of the Invention
The present invention relates to an engine that is selectively operable in two- and four-cycle modes depending on the rotational speed of the engine and the load on the engine.
2. Description of the Prior Art
Conventional reciprocal-piston engines are roughly classified into two-cycle engines in which intake, compression, power, and exhaust strokes occur while the piston reciprocates once, i.e., the crankshaft makes one revolution, and four-cycle engines in which intake, compression, power, and exhaust strokes occur while the piston reciprocates twice, i.e., the crankshaft makes two revolutions.
In the two-cycle engines, the intake and exhaust strokes are effected around the bottom dead center of the piston, and the power stroke is effected each time the crankshaft makes one revolution. Therefore, the two-cycle engines suffer lower fluctuations of the rotational speed of the output shaft, and can produce a higher torque.
In the four-cycle engines, since intake and exhaust strokes occur independently of each other, the exhaust gases are fully replaced with a new air-fuel mixture. Accordingly, the four-cycle engines are more advantageous than the two-cycle engines in that the exhaust gases are cleaner, and the fuel consumption in high-speed engine operation is lower.
Inasmuch as the two- and four-cycle engines have different operating characteristics, an engine which can operate as a two-cycle engine in a low-speed operation range and as a four-cycle engine in a high-speed operation range is expected to offer the advantages of both two- and four-cycle engines.
However, it is highly difficult to change the timing to open and close the intake and exhaust valves while the engine is in operation. There has not been available an engine having a practically feasible mechanism for modifying the timing to open and close the intake and exhaust valves.